Over the recent years, a fuel cell has been focused as a power source that is excellent in terms of operation efficiency and environmentality. The fuel cell outputs the electric power in response to a request of a drive motor by controlling a quantity of supply of fuel gas, however, there is a case in which responsibility of the output electric power decreases due to a delay of response of the gas supply quantity, and a secondary battery might be mounted for compensation thereof. This secondary battery accumulates a regenerative energy generated when the drive motor is decelerated and the electric power generated by the fuel cell, and discharges the accumulated energy for the purpose such as compensating the decrease in responsibility of the fuel cell and increasing the output of the whole fuel cell system.
Herein, the fuel cell system, in which a power source is built up by connecting the fuel cell to the secondary battery in parallel, schemes to take a combined use of the fuel cell and the secondary battery in such a way that a DC-DC converter converts an output voltage of the fuel cell and an output voltage of the secondary battery. Then, a technology (refer to, e.g., Patent document 1) is disclosed, which controls drive of the DC-DC converter executing a voltage conversion of an output given from the fuel cell on the basis of a request output from the drive motor while taking a switching loss due to this DC-DC converter into consideration. According to this technology, if the request output of the drive motor is equal to or smaller than a predetermined threshold value, the DC-DC converter is set in an electrically direct-connected state, and the drive motor is supplied with the output of the fuel cell in preference to the output of the secondary battery.    [Patent document 1] Japanese Patent Laid-Open Publication No. 2007-184243    [Patent document 2] Japanese Patent Laid-Open Publication No. 2006-310271    [Patent document 3] Japanese Patent Laid-Open Publication No. 2005-348530    [Patent document 4] Japanese Patent Laid-Open Publication No. 2007-209161    [Patent document 5] Japanese Patent Laid-Open Publication No. 2003-235162    [Patent document 6] Japanese Patent Laid-Open Publication No. 2000-36308